1. Field Of The Invention
The invention relates to a new process for the production of carboxylic acid chlorides of aromatic nitrogen heterocycles of the series of compounds of general formulas: ##STR3## by chlorination of the corresponding hydroxy carboxylic acids of aromatic nitrogen heterocycles from the series of compounds of the general formulas: ##STR4## In the above general formulas I to VI, R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5, R.sub.6, R.sub.7 and R.sub.8 are the same or different and each is a hydrogen atom or a halogen atom.
These carboxylic acid chlorides of aromatic nitrogen heterocycles, such as, 6-chloropicolinic acid chloride, are important intermediate products for the production of herbicides [European Published Patent Application No. 447,004].
2. Background Art
The direct reaction of 6-hydroxypicolinic acid to 6-chloropicolinic acid chloride has not yet been described in the literature. Only the chlorination of 6-hydroxypicolinic acid to 6-chloropicolinic acid with phosphorous pentachloride in phosphoroxy chloride is known [Ber. D. Chem. ges., 45, (1912), page 2461]. 6-Chloropicolinic acid chloride can then be produced only by another chlorination of 6-chloropicolinic acid with, for example, thionyl chloride. A drawback of this process is the fact that under these conditions the product can only be produced by a 2-stage synthesis and that large amounts of phosphoric acid accumulate in the first stage as waste product in the working-up of 6-chloropicolinic acid as the intermediate product.
Also, the production of 6-chloropicolinic acid chloride, starting from 6-chloro-2-methylpyridine, is known [European Published Patent Application No. 032516]. A great drawback of this process is the fact that the feedstock 6-chloro-2 -methylpyridine must first be oxidized in a first stage with potassium permanganate to 6-chloropicolinic acid, the latter compound is then converted with thionyl chloride in the second stage into 6-chloropicolinic acid chloride, and the product is obtained in a moderate yield.